1. Field of the Invention
This invention relates to a process for preparing polyethylene plexifilamentary film-fibril strands. More particularly, the invention concerns an improved process in which the strand is flash-spun from mixtures of polyethylene, an organic solvent and water.
2. Description of the Prior Art
Blades and White, U.S. Pat. No. 3,081,519 describes a flash-spinning process for producing plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash spinning causes the liquid to vaporize and thereby cool the plexifilamentary film-fibril strand that forms from the polymer. Preferred polymers include crystalline polyhydrocarbons such as polyethylene and polypropylene.
According to U.S Pat. No. 3,081,519 the following liquids are useful in the flash-spinning process: aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids. The patent further states that the flash-spinning solution additionally may contain a dissolved gas, such as nitrogen, carbon dioxide, helium, hydrogen, methane, propane, butane, ethylene, propylene, butene, etc. Preferred for improving plexifilament fibrillation are the less soluble gases, i.e., those that dissolve to a less than 7% concentration in the polymer solution under the spinning conditions.
Flash spinning a polyolefin discrete fiber from a polymer dissolved in a solvent with water added in quantities sufficient to form an emulsion or inverse emulsion is known. For example, Kozlowski U.S. Pat. No. 4,054,625 teaches a process of manufacturing discrete fibers from water and a solution of polymer in an organic solvent and water. Critical to the process of Kozlowski, is that the water is present in an amount such that it constitutes a discontinuous phase dispersed as discrete droplets throughout the polymer solution. This "inverse emulsion" is then flash spun to form discrete fibers. Water concentrations of 40 to 50%, far exceeding the solubility of water in the organic solvent, are preferred for the process even though more care in mixing the solution must be exercised to ensure that the water is the discontinuous phase.
Commercial spunbonded products made from polyethylene plexifilamentary film-fibril strands have been successfully produced with the polyethylene being flash-spun from trichlorofluoromethane. Although trichlorofluoromethane has been used extensively for this purpose, the escape of such a halocarbon into the atmosphere has been implicated as a source of depletion of the earth's ozone. A general discussion of the ozone-depletion problem is presented, for example, by P. S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting Halocarbons", Chemical & Engineering News, pages 17-20 (Feb. 8, 1988).
This invention provides an improved process for preparing polyethylene plexifilamentary film-fibril strands. The strand is spun from a non-chlorofluorocarbon mixture of polyethylene, an organic solvent and water.